The long range goal of these studies is to understand and possibly control the role of submicroscopic membrane domains in fertilization. Principally, two techniques will be used to study the nature of membrane domains: nonradiative fluorescence resonance energy transfer and label-freeze fracture electron microscopy. Two biological fertilization systems will be studied: the sea urchin and the mouse. We propose to critically examine the nature of the domain organization in the membranes involved in fertilization, the role of these domains in fertilization, and whether the ensemble of domains can be manipulated advantageously either to prevent or to promote fertilization. Specifically, we will ask: 1) What are the nature of the domains present in the membranes of the fertilization system? a) Are lipids sequestered into domains? b) Are proteins sequestered into domains? c) Do domains occur in isolated membrane vesicles? d) Do domains occur in the absence of proteins? e) What are the sizes of these domains? 2) Do domains play a role in the physiological processes of fertilization? a) Hyperactivation, b) The acrosome reaction, c) Sperm-egg fusion, d) The cortical reaction. 3) Can the ensemble of domains be altered so as to inhibit and/or enhance the fertilization processes?